The present invention relates to the application of liquid coatings to continuously moving objects, and more particularly to the non-contact deposition of functional coatings to a metal strip moving along a horizontal process line.
One type of device for automatically coating a moving metal strip with a fluid such as oil is disclosed in U.S. Pat. No. 2,994,618 issued to G. F. Landgraf. The device disclosed in this patent is primarily designed to apply micro-thin films of oil to light gauge container materials. For example, it can be used to apply lubricants on electrolytic tinning lines, chrome lines and recoil lines. The lubricant is applied to the metal strip as it is moving in a vertical direction, which may require a change in direction of the metal but is easily provided for in a line of light gauge material. Typically, in depositing operations in which the Landgraf device is used, the metal strip moves at a speed in the range of 400-2000 feet per minute, and the coating weight of the lubricant is in the range of 0.2-1.2 mg/ft.sup.2 per side.
The Landgraf device has met with considerable success in the field of depositing oil coatings to light gauge metals, and it is desirable to provide an apparatus that is capable of achieving similar results in process lines for heavier and thicker materials, such as in various steel strip treatment and forming applications.
There are a variety of reasons for applying functional types of coatings to steel strips. One of the most significant of these reasons is for corrosion protection. Surface oxidation or corrosion that occurs prior to the time the strip product reaches its final destination, or while it is in storage for fabrication, can result in substantial losses, and it is therefore necessary to protect against such diminishing of the product quality.
Furthermore, during high production metal forming and fabrication operations, such as cold rolling, surface lubrication of the metal is critical. The preciseness of the operations requires close control of the amount and type of lubricant that is applied. For example, too small an amount of lubricant can result in excessive friction wear on tools and production equipment. Conversely, too much lubricant has an adverse effect on the metal forming equipment, such as clogging dies and tools. It also inhibits subsequent chemical cleaning of the metal strip, and makes handling of the product difficult.
In addition, it is desirable to apply functional coatings to steel strips for surface pretreatment and cosmetic reasons.
In the past, coatings have been typically applied to steel strips by means of a distribution pipe disposed over the moving strip. The pipe has holes in it which allow the coating material to drip onto the strip, and cloth wiper rolls are used to meter the amount of oil that remains on the strip. Mechanical spray nozzles have also been used to apply coatings. Disadvantages associated with these techniques include the waste of coating material that results, the environmental and safety problems that are generated when excess coating material is released into the working environment, and the unpredictability and nonuniformity of the coating that is applied. In addition, they may be limited to applying the coating to one side of the strip, and they may not be adjustable for varying strip widths.
An alternative to the use of spray or drip type coating has been to apply the liquid by an electrostatic process in which the material to be applied is electrically charged. However, most electrostatic coaters that are used in continuous process lines are adapted to apply only non-conductive types of coatings, such as petroleum based oils and organic lubricants. Recently, a number of conductive materials having preferred functional characteristics have been developed. For example, aqueous based vapor phase rust inhibitors have become popular, and it is desirable to apply these through an electrostatic process.
The previously described Landgraf device, while well suited to the non-contact coating of light metals, is not readily applicable to the steel strip process. For example, it is most desirable to provide a strip coating device in a horizontal section of the steel process line, whereas the Landgraf device is designed for operation in a vertical pass section. It is not generally desirable to change the direction of heavy gauge steel strip, as opposed to light gauge metals. Furthermore, and perhaps more significantly, the typical coat weights that are required in steel processing lines (e.g. 50-400 mg/ft.sup.2 per side) are substantially greater than those encountered in light gauge metal processing. The uniform application of such coat weights at line speeds that can vary between 50-5000 ft/min pose a requirement for a different type of fluid application system than that used in light gauge systems. In other words, it is not simply a matter of increasing the operating parameters of the system to obtain the desired coat weights and still achieve the desired objects.